class_docstr: An encapsulation of a change to be made to the tree.
  Constructed of three components
    - the node to be changed
    - what it should be changed to
    - a string name of the kind of change (normally grow or prune)
code: def __str__(self):
      return "{} - {} => {}".format(
          self.kind, self.existing_node, self.updated_node
      )
  
file_path: imodels/imodels/experimental/bartpy/mutation.py
imports: from .node import TreeNode, DecisionNode, LeafNode
label_desc: Use image node_id 2 for calling the TreeMutation obj's underlying member method code with example usage: obj.__str__() and returns: str